Electrical discharge machining (EDM) processes are generally divided into two categories, sinking-type EDM and traveling-wire EDM. In the sinking-type EDM process, the tool electrode is a simple or formed solid electrode designed to form a cavity complementary in shape thereto in a workpiece. In this process, the workpiece is immersed in the machining liquid commonly constituted by a hydrocarbon liquid such as kerosene. A worktank is used to retain the hydrocarbon machining liquid and the workpiece is immersed therein and positioned sufficiently below the surface of this liquid in the worktank. The tool electrode is commonly formed with one or more fluid passages therein through which the machining liquid is supplied to the machining gap. Alternatively or in addition, one or more nozzles are disposed in the region of the tool electrode or the workpiece and used to direct the machining liquid into the machining gap. It has been recognized that this process entails a danger of fire because of the inflammability of the hydrocarbon liquid. When electrical discharges effected through the hydrocarbon liquid are exposed to air, fire tends to develop and may result in property damage. Furthermore, the hydrocarbon liquid upon decomposition by electrical discharges produces gases and mists which tend to pollute the environmental atmosphere. While the danger of fire may be alleviated by adding certain chemicals to the hydrocarbon liquid, this adds to cost and may result in a significant reduction in the machining efficiency.
In the traveling-wire EDM process, the tool electrode is a thin, continuous wire which is axially transported to continuously traverse the workpiece exposed to air. The machining gap formed between the traveling wire electrode and the workpiece needs to be flushed with the machining liquid which, however, with the workpiece always exposed to air, can in no way be an inflammable hydrocarbon liquid as in the sinking-type EDM process. Thus, in the traveling-wire EDM process, it has been the common practice to employ a water liquid as the machining liquid.
With the traveling-wire process gaining increasing popularity attempts have been made to apply a water liquid, which previously found its sole use in that process, to the sinking-type process. It has been found, however. that the water liquid when used in the sinking-type process is incapable of yielding a mirror-finish machined surface as required in an ultra-fine machining range and further is unsuitable for use with "no wear" or "low-wear" machining requirements. Moreover, it has been observed that with the tool machining area reduced, say, to less than several hundred square millimeters, the water machining liquid seldom offers the required machining stability and efficiency in the sinking-type process.